The fact that Xmiplay wouldn't honor their warranty on a two-week old frame would scare me off from doing any business with them in the future. It also definitely doesn't help their case that they sent you the wrong chainstay. A LBS might be able to create some kind of temporary fix for your pivot play issues, but you might just as well served going to a hardware store and finding some kind of nylon spacer that will press it together.
As for differences between the two frames, the FS029 is not the same as the FM258. The FS029 has a 2.1 degree slacker HTA, a 180mm minimum rear disc brake rotor, shorter reach, and different pivot assemblies, to name a few differences to the FM258. The FS029 is also made by XM Carbonspeed, which has a reputation for good communication and warranty policy. You can find the geometry chart here
http://www.xmcarbonspeed.com/Productinfo.asp?f=1527. Both frames have a "flex pivot" where the rear pivot would be on the 036, so they would both be much stiffer.
Although I do not own either of those frames, I have seen a few build threads on the FM258 variation, the FM058. I would go that route given the documentation that exists on the frames. The Hongfu frame seems very solid, and is also being sold by the European companies Megamo and Price. Russian World Cup XC racer Anton Sintsov appears to be racing on an FM258 frame currently, and Swiss rider Martin Fanger just completed the Absa Cape Epic on an FM058.
